Discharge device and image forming apparatus

ABSTRACT

A discharge device comprises a discharge roller unit including a first discharge roller and a second discharge roller, and deforming a sheet on which an image is formed, the sheet being deformed such that the sheet is recessed toward one of the rollers when the sheet is fed between the first discharge roller and the second discharge roller.

INCORPORATION BY REFERENCE

This application is based upon and claims the benefit of priority from the corresponding Japanese Patent application No. 2008-221217 and 2009-104128 filed Aug. 29, 2008 and Apr. 22, 2009 respectively, the entire contents of which are incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to discharge devices. The present invention also relates to image forming apparatuses including the discharge device.

2. Description of the Related Art

In an electrophotographic image forming apparatus, a sheet of paper is fed to an image forming section from a sheet storage section in which sheets of paper are stacked. An image is then formed on a surface of the sheet. The sheet having the image formed thereon is discharged by a sheet discharge section. In the image forming section, the surface of a photosensitive member is uniformly charged by a charging unit, and is subjected to an exposure process performed by an exposure unit pursuant to a signal transmitted from an information processing terminal. As a result, a latent image is formed on the surface of the photosensitive member. Next, the latent image is developed into a toner image by a developing unit. The toner image, formed on the surface of the photosensitive member, is transferred by a transferring unit onto the sheet that is fed from the sheet storage section. Then, the sheet on which the toner image is formed is fed to a fixing unit. The fixing unit applies heat and pressure to the sheet fixing the toner image. The sheet on which the toner image is fixed is discharged from the apparatus by the sheet discharging unit. The thus-discharged sheet is placed on a stacking tray located outside the apparatus.

As described above, in the process of forming the toner image on the sheet, the fixing unit applies heat to the toner image so as to fix the toner image on the sheet. Therefore, when multiple sheets are successively discharged from the apparatus after the fixing step, heated sheets are stacked on the stacking tray one after the other. Therefore, heat accumulates in the stack of sheets.

The discharged sheets are naturally cooled so that the toner on the surfaces of the sheets is sufficiently solidified. However, if heat accumulates in the stack of sheets, the toner on the sheets does not quickly solidified and remains partially molten. If the molten toner is cooled and solidified while the weight of the stacked sheets is applied thereto, there is a risk that the sheets will strongly stick to each other. In addition, when the sheets that are stuck to each other are separated, there is a risk that the toner will be removed from the sheets and adhere to the back sides of the following sheets. Thus, when heat accumulates in the stack of sheets, there is a problem in that the image quality will be reduced and/or the sheets will be stained.

Various techniques have been proposed to solve these problems. An example of one such technique is a method of reducing the temperature of the sheets after the fixing step via a cooling roller.

In an example of a known image forming apparatus, the cooling roller is located between the fixing unit and the stacking tray to reduce the temperature of the sheets after the fixing step. In this apparatus, as each sheet passes the cooling roller after the fixing step, the temperature of the sheet is reduced by the cooling roller. Therefore, even when the sheets are successively discharged outside of the apparatus after the fixing step, heat can be prevented from accumulating in the stack of sheets on the stacking tray.

However, as the temperature of each sheet is reduced by the cooling roller, the temperature of the cooling roller increases. A blower fan is used to reduce the temperature of the cooling roller by blowing air toward the cooling roller. Thus, in the known image forming apparatus, the temperature of the cooling roller is controlled via a blower fan to prevent the increase in the temperature of the cooling roller.

However, in this type of image forming apparatus, a cooling roller and a blower fan must be provided to reduce the temperature of the sheets after the fixing step. Therefore, space for the cooling roller and the blower fan must be provided, and the additional cost of using a cooling roller and a blower fan is incurred. In addition, if the speed of air ejected from the blower fan to cool the cooling roller is too great, there is a risk that sheet feeding performance will be reduced.

SUMMARY

The present invention provides discharge devices capable of reducing the temperature of heated sheets without using additional components or devices, such as a cooling roller and a blower fan. Another advantage of the present invention is to provide image forming apparatuses including the discharge device capable of reducing the temperature of heated sheets without using additional components or devices, such as a cooling roller and a blower fan.

According to an embodiment of the present invention, a discharge device is provided including a discharge roller unit. The discharge roller unit includes a first discharge roller and a second discharge roller. The discharge roller unit deforms a sheet on which an image is formed, the sheet being deformed such that the sheet is recessed toward one of the rollers when the sheet is fed between the first discharge roller and the second discharge roller.

Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

In the accompanying drawings:

FIG. 1 is a schematic sectional view of a color printer according to an embodiment of the present invention;

FIG. 2 is a diagram illustrating the structure of discharge roller according to an embodiment of the present invention;

FIG. 3 is a schematic top view of a sheet which has passed through the discharge roller according to an embodiment of the present invention;

FIG. 4 is a schematic diagram illustrating the state in which sheets are stacked on a stacking tray according to an embodiment of the present invention;

FIG. 5 is a diagram illustrating the manner in which roller members are attached according to an embodiment of the present invention;

FIG. 6 is a diagram illustrating the structure of a thrust mechanism according to an embodiment of the present invention;

FIGS. 7A and 7B are schematic diagrams illustrating a sheet discharged by discharge rollers according to an embodiment of the present invention; and

FIG. 8 is a diagram illustrating the structure of a mechanism for changing the distance between shafts of discharge roller according to an embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram illustrating the structure of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus shown in FIG. 1 is a tandem color printer 1 which forms an image on a sheet of paper based on the image information.

The color printer 1 includes a printer body 100 including: an image forming section 2 for forming a color image; toner containers 3M, 3Y, 3C, and 3B which correspond to respective colors; a conveying belt 4 for conveying a sheet of paper, which is an example of a recording medium; first to fourth transfer rollers 5M, 5Y, 5C, and 5B for transferring toner images of respective colors onto the sheet fed thereto; a sheet feeding section 6 which stores and feeds the sheet; a conveying path 22 which functions as a feed mechanism for feeding the sheet; a fixing unit 7 which fixes the toner images transferred onto the sheet; and a discharge section 8. The image forming section 2 includes image forming units 2M, 2Y, 2C, and 2B for the respective colors.

The image forming units 2M, 2Y, 2C, and 2B form magenta, yellow, cyan, and black toner images, respectively. The image forming units 2M, 2Y, 2C, and 2B are located along the conveying belt 4, in that order, from an upstream side in a direction in which the sheet is fed. Each of the image forming units 2M, 2Y, 2C, and 2B includes a photosensitive drum 10 which includes a photosensitive layer made of amorphous silicon or the like and which serves as an image bearing member, a charging unit 11, an exposure unit 12, a developing unit 13, and a cleaning unit 14. The charging unit 11, the exposure unit 12, the developing unit 13, and the cleaning unit 14 are located around each of the photosensitive drums 10, in that order, along the rotating direction of the photosensitive drum 10.

The toner containers 3M, 3Y, 3C, and 3B contain magenta, yellow, cyan, and black toners. The toner containers 3M, 3Y, 3C, and 3B are attached to the printer body 100. The toner containers 3M, 3Y, 3C, and 3B are located above the image forming units 2M, 2Y, 2C, and 2B, respectively, and are located along the conveying belt 4 in that order from an upstream side in the direction in which the sheet is fed.

The conveying belt 4 is stretched between a driving roller 20 and a following roller 21, and is driven in a circulating manner. A sheet of paper is fed from the sheet feeding section 6 through the conveying path 22 and is fed by the conveying belt 4 while being stuck thereto. The conveying belt 4 feeds the sheet in a substantially horizontal direction such that the sheet passes through the sections between the respective photosensitive drums 10 and the first to fourth transfer rollers 5M, 5Y, 5C, and 5B. The driving roller 20 and the following roller 21 are rotatably attached to the printer body 100.

The first to fourth transfer rollers 5M, 5Y, 5C, and 5B are rotatably attached to the printer body 100, and are located so as to face the photosensitive drums 10 included in the image forming units 2M, 2Y, 2C, and 2B, respectively. Each of the first to fourth transfer rollers 5M, 5Y, 5C, and 5B can be moved between an upper transfer position and a lower release position by a solenoid 23. When the first to fourth transfer rollers 5M, 5Y, 5C, and 5B are in the transfer positions thereof, the conveying belt 4 is nipped between the first to fourth transfer rollers 5M, 5Y, 5C, and 5B and the respective photosensitive drums 10. In other words, the sheet fed by the conveying belt 4 comes into contact with the photosensitive drums 10. When the first to fourth transfer rollers 5M, 5Y, 5C, and 5B are at the release positions thereof, the conveying belt 4 is separated from the photosensitive drums 10.

The conveying path 22 includes a guide member 22 a which guides the sheet when the sheet is fed, a plurality of pairs of rollers 22 b, and a pair of registration rollers 25. The registration rollers 25 are located at the downstream end of the conveying path 22, and serve to synchronize the process of forming the toner images in the image forming section 2 and the process of feeding the sheet. A bias roller 26 is located between the registration rollers 25 and the first transfer roller 5M. The bias roller 26 is located so as to face (to be in contact with) the following roller 21, around which the conveying belt 4 is stretched, at an upper section of the following roller 21. The bias roller 26 applies a bias voltage to the leading end or the entire area of the sheet, thereby causing the sheet to stick to the conveying belt 4. To simplify the figures, only two pairs of rollers 22 b are shown in FIG. 1 and the other pairs of rollers 22 b are omitted.

The fixing unit 7 includes a fixing roller 7 a and a pressure roller 7 b. The fixing roller 7 a is rotatably attached to the printer body 100. The fixing roller 7 a serves to heat the toner transferred to the sheet and fix the toner images on the sheet. The fixing roller 7 a includes a heater. The pressure roller 7 b is rotatably attached to the printer body 100 such that the pressure roller 7 b is in pressure contact with the fixing roller 7 a. When the sheet to which the toner images are transferred is fed between the pressure roller 7 b and the fixing roller 7 a, the toner on the sheet is heated by the fixing roller 7 a and pressed by the pressure roller 7 b. As a result, the toner transferred to the sheet is fixed on the sheet. The feed rate at which the sheet is fed by the rotation of the fixing roller 7 a is set at a predetermined speed. In an embodiment, the speed is set to 250 mm/s.

The discharge section 8 is located in the printer body 100. The discharge section 8 feeds the sheet delivered from the fixing unit 7, that is, the sheet on which the toner images are fixed, and discharges the sheet to a stacking tray 9. The discharge section 8 includes a pair of discharge roller units 81, a plurality of pairs of conveying rollers 82, a guide member 83, and the stacking tray 9. The pair of discharge roller units 81 and the plurality of pairs of conveying rollers 82 are driven by driving units M1 and M2 including a first motor and a second motor, respectively. An example wherein the driving units M1 and M2 are not included in the discharge section 8 will now be explained. However, the driving units M1 and M2 may also be included in the discharge section 8.

The plurality of pairs of conveying rollers 82 nip and feed the sheet on which the toner is fixed. The conveying rollers 82 are connected to the first motor M1 and are rotated by the first motor M1. The guide member 83 guides the sheet on which the toner is fixed when the sheet is fed. The plurality of pairs of conveying rollers 82 and the guide member 83 feed the sheet on which the toner is fixed to the pair of discharge roller units 81. To simplify the figures, only one pair of conveying rollers 82 are shown in FIG. 1, and the other pairs of conveying rollers 82 are omitted.

Each discharge roller unit 81 is rotatably attached to the printer body 100. In the following description, the upper discharge roller unit 81 in FIG. 1 is called a first discharge roller 181 and the lower discharge roller unit 81 is called a second discharge roller 281.

As shown in FIG. 2, the first discharge roller 181 includes a shaft member 181 a and a plurality of roller members 181 b. The second discharge roller 281 includes a shaft member 281 a and a plurality of roller members 281 b. The roller members 181 b and 281 b are attached to the shaft members 181 a and 281 a, respectively, in a non-rotatable manner. The centers of the shaft members 181 a and 281 a and the centers of the roller members 181 b and 281 b coincide with each other. In the first discharge roller 181, the roller members 181 b are attached to the shaft member 181 a in a non-rotatable manner at positions where the roller members 181 b face the roller members 281 b of the second discharge roller 281. The sheet on which the toner is fixed is fed while being nipped between the roller members 181 b of the first discharge roller 181 and the roller members 281 b of the second discharge roller 281.

More specifically, the first discharge roller 181 and the second discharge roller 281 are rotatably attached to the printer body 100 such that they are arranged in a vertical direction and are in pressure contact with each other. The first discharge roller 181 feeds the sheet while being in contact with the top surface of the sheet. The first discharge roller 181 includes five roller members 181 b. In an embodiment, the shaft member 181 a has a diameter of 5 mm and is made of metal. In an embodiment, the roller members 181 b have a cylindrical shape with a diameter of 15 mm and a width of 5 mm, and are made of polyacetal resin. The five roller members 181 b are attached to the shaft member 181 a in a non-rotatable manner and are arranged with predetermined intervals therebetween in the axial direction of the shaft member 181 a.

The second discharge roller 281 feeds the sheet while being in contact with the bottom surface of the sheet. In an embodiment, the shaft member 281 a has a diameter of 5 mm and is made of metal. In an embodiment, the roller members 281 b have a diameter of 15 mm and a width of 20 mm, and are made of ethylene-propylene-diene monomer (EPDM) rubber. The second discharge roller 281 includes five roller members 281 b. In general, the roller members 281 b are preferably made of EPDM rubber having a hardness, in an embodiment, of 50° to 80° in terms of the JIS A hardness. The five roller members 281 b are attached to the shaft member 281 a in a non-rotatable manner and are provided with predetermined intervals therebetween in the axial direction of the shaft member 281 a. The shaft member 281 a of the second discharge roller 281 is connected to the second motor M2 and is rotated by the second motor M2.

The rotational speed of the second motor M2 is controlled such that the ratio of the sheet-conveying speed of the second discharge roller 281 according to an embodiment to the sheet-conveying speed of the fixing roller 7 a (reference speed) is preferably in the range of 1.00 to 1.02. In an embodiment, the feed rate at which the sheet is fed by the rotation of the second discharge roller 281 is set to 252 mm/s. Thus, the rotational speed of the second motor M2 is controlled such that the sheet-conveying speed of the second discharge roller 281 (252 mm/s) is greater than the sheet-conveying speed of the fixing roller 7 a (250 mm/s). The plurality of pairs of conveying rollers 82 are also driven such that the feed rate thereof is equal to that of the discharge roller units 81.

Either the first discharge roller 181 or the second discharge roller 281 is urged against the other by an elastic member, for example, using a spring member (not shown). In other words, the first discharge roller 181 is urged in a direction that crosses the shaft member 181 a of the first discharge roller 181, in a direction toward the second discharge roller 281. Therefore, the roller members 181 b of the first discharge roller 181 and the roller members 281 b of the second discharge roller 281 are in contact with each other while being urged against each other, so that the sheet can be nipped between the roller members 181 b of the first discharge roller 181 and the roller members 281 b of the second discharge roller 281.

In an embodiment, the shaft member 181 a of the first discharge roller 181 is urged by a spring member in a direction towards the shaft member 281 a of the second discharge roller 281 with a force of 10.0 N. Since five pairs of roller members are arranged, a force of 2.0 N is applied to the pressure contact surfaces of each pair of roller members 181 b and 281 b, which are in contact with each other. A portion of the sheet is retained by the above-mentioned force. It is believed that the force (pressure contact force) applied to the pressure contact surfaces of each pair of roller members 181 b and 281 b is greater than the pressure contact force applied in the prior art apparatus; the contact force applied to each pair of roller members in the prior art (reference value) is about 0.4 N to 1.0 N.

In an embodiment, the roller members 281 b are made of rubber, and the roller members 181 b are made of resin. However, the materials of the roller members 181 b and 281 b are not particularly limited as long as the hardness of the roller members 181 b in the first discharge roller 181 (first hardness) differs from the hardness of the roller members 281 b in the second discharge roller 281 (second hardness). More specifically, the hardness of the roller members 181 b in the first discharge roller 181 (first hardness) is greater than the hardness of the roller members 281 b in the second discharge roller 281 (second hardness). The hardness of the roller members 181 b is set to be equal to or greater than 90° in terms of the JIS A hardness.

Due to the pressure contact force and the difference in hardness, which are set as described above, the sheet that passes through the plurality of pairs of roller members 181 b and 281 b can be deformed such that the sheet is recessed in a direction toward roller members 281 b. More specifically, due to the pressure contact force and the difference in hardness, as shown in FIG. 3, tracks H (passage marks) of the roller members 181 b can be formed on the sheet when the sheet passes through the roller members 181 b and 281 b.

As shown in FIG. 6, the shaft member 181 a of the first discharge roller 181 is connected to the second motor M2, and is reciprocated in an axial direction by the second motor M2. Thus, the first discharge roller 181 is reciprocated in the axial direction by the second motor M2. At this time, the first discharge roller 181 is reciprocated by the second motor M2 in the axial direction at a period different from a period of a rotation at which each sheet is fed. Therefore, when a plurality of sheets are stacked on the stacking tray 9, the tracks H of the roller members 181 b shown in FIG. 3 are formed at different positions for each of the sheets. Thus, the sheets can be prevented from being stacked so that the tracks H on the sheets are at the same positions. As a result, spaces can be provided between the sheets and the heat in the stack of sheets can be quickly dissipated.

Now, a thrust mechanism, that is, a mechanism for reciprocating the shaft member 181 a of the first discharge roller 181 in the axial direction, will be described with reference to FIG. 6.

The shaft member 281 a of the second discharge roller 281 is restrained from moving in the axial direction with respect to the shaft-member-supporting side plates 101, which are provided in the printer body 100. The shaft member 281 a is rotatably retained by the shaft-member-supporting side plates 101. The shaft member 181 a of the first discharge roller 181 is rotatably retained by the shaft-member-supporting side plates 101 such that the shaft member 181 a can slide in the axial direction. An urging spring 300, which functions as an urging member, is fitted to the shaft member 181 a of the first discharge roller 181 at an end thereof. Accordingly, the shaft member 181 a of the first discharge roller 181 is urged in a direction from one of the shaft-member-supporting side plates 101 (left side plate in FIG. 6) to the other shaft-member-supporting side plate 101 (right side plate in FIG. 6). The other end of the shaft member 181 a of the first discharge roller 181 is in contact with a side surface of a cam gear 312, which will be described below.

The pin 312 a is fixed to one of the shaft-member-supporting side plates 101, and the cam gear 312 is rotatably retained by the pin 312 a such that the cam gear 312 is restrained from moving in the axial direction. The cam gear 312 has a slope 312 b on one side (side facing the side plate 101) thereof, and the height of the slope 312 b in the axial direction varies continuously. The shaft member 181 a of the first discharge roller 181 is retained such that the axial center thereof is on a circumference near the outermost periphery of the cam gear 312. The other end of the shaft member 181 a of the first discharge roller 181 is pressed against the slope 312 b by the urging spring 300.

An input gear 310, which is rotated by the second motor M2, meshes with a drive gear 311 fixed to the shaft member 281 a and the cam gear 312 which serves to slide the shaft member 181 a. When the input gear 310 is rotated by the second motor M2, the second discharge roller 281, to which the drive gear 311 is fixed, is also rotated. Thus, the roller members 281 b included in the second discharge roller 281 rotate. As a result, the roller members 181 b of the first discharge roller 181, which are in pressure contact with the roller members 281 b, are rotated by the rotation of the roller members 281 b, and, accordingly, the first discharge roller 181 is rotated.

In addition, when the input gear 310 is rotated by the second motor M2, the cam gear 312 is also rotated. When the cam gear 312 is rotated, the height of the slope 312 b at the position of the axial center of the first discharge roller 181 changes. Accordingly, the shaft member 181 a of the first discharge roller 181, which is in contact with the cam gear 312, slides in the axial direction in accordance with the change in the height of the slope 312 b. Thus, the shaft member 181 a of the first discharge roller 181 slides in the axial direction while being rotated. In an embodiment, the shape of the slope 312 b is set such that the shaft member 181 a of the first discharge roller 181 reciprocates once while the cam gear 312 rotates one turn. The period of reciprocation can be adequately adjusted by changing the ratio of the number of teeth on the cam gear 312 to the number of teeth on the input gear 310.

A control board (not shown) for controlling the driving units M1 and M2 is attached to the printer body 100. The driving forces generated by the driving units M1 and M2 are controlled by a CPU mounted on the control board. For example, the sheet-conveying speed of the first motor M1 and the speed at which the first discharge roller 181 and the second discharge roller 281 are driven by the second motor M2 are controlled by the CPU based on the programs stored in the memory, such as a ROM, mounted on the control board.

The distance which the first discharge roller 181 is moved in the axial direction is set to a predetermined distance. In an embodiment, the distance is set at 0.4 mm. The reciprocation rate at which the first discharge roller 181 reciprocates in the axial direction is set to a predetermined rate. In an embodiment, the reciprocation rate is set to three times per second.

Tables 1 and 2 show the result of an experiment in which occurrences of sheet wrinkles, sticking of the sheets, and offset were observed. The offset is a phenomenon in which part of a toner image on a sheet A, that is discharged to the stacking tray 9, transfers onto the back side of another sheet B in the stacking tray 9 while the toner image on the sheet A is not yet sufficiently solidified. Since a part of the toner image on the sheet A is separated from the sheet A, the image quality of the toner image on the sheet A is reduced. The print quality of the sheet B is also reduced since the back side thereof is stained.

In the experiment, the average depth of the recesses formed in the sheets was 12 μm. The depth of the recesses formed in each sheet was measured by the following method. First, each sheet is held at one side thereof such that the sheet hangs in a container which is set such that no air flow occurs therein. Then, a surface of the sheet is scanned in a direction perpendicular to the sheet with a measurement laser beam emitted from a laser displacement sensor (LK-80 produced by Keyence Corporation). The depth of the recesses in the sheet can be determined from the result of the scanning process using the laser beam.

It has been determined from the experiment that no sheet wrinkles, sticking of the sheets, or offset occurs when the distance by which the first discharge roller 181 is moved in the axial direction is set to 0.4 mm and the reciprocation rate of the first discharge roller 181 in the axial direction is set to three to five times per second.

TABLE 1 SHEET DISTANCE RECIPROCATION RATE WRINKLES 0.2 mm 8 TIMES PER SEC NONE 5 TIMES PER SEC NONE 3 TIMES PER SEC NONE 1 TIMES PER SEC NONE 0.4 mm 8 TIMES PER SEC FEW 5 TIMES PER SEC NONE 3 TIMES PER SEC NONE 1 TIMES PER SEC NONE 0.6 mm 8 TIMES PER SEC MANY 5 TIMES PER SEC MANY 3 TIMES PER SEC FEW 1 TIMES PER SEC NONE 0.8 mm 8 TIMES PER SEC MANY 5 TIMES PER SEC MANY 3 TIMES PER SEC MANY 1 TIMES PER SEC FEW

In Table 1, “NONE” means that no sheet wrinkles were found in any of the sheets, “FEW” means that no sheet wrinkles were found in 98% or more of the sheets, and “MANY” means that sheet wrinkles were found in more than 2% of the sheets. In the “STICKING” and “OFFSET” columns in Table 2, “NONE” means that no problems occurred in any of the printed sheets, “FEW” means that no problems occurred in 98% or more of the printed sheets, and “MANY” means that problems occurred in more than 2% of the printed sheets.

TABLE 2 DISTANCE RECIPROCATION RATE STICKING OFFSET 0.2 mm 5 TIMES PER SEC NONE NONE 3 TIMES PER SEC FEW NONE 1 TIMES PER SEC MANY MANY 0.4 mm 5 TIMES PER SEC NONE NONE 3 TIMES PER SEC NONE NONE 1 TIMES PER SEC FEW NONE 0.6 mm 5 TIMES PER SEC NONE NONE 3 TIMES PER SEC NONE NONE 1 TIMES PER SEC NONE NONE

In the experiment, VM (Nippon Paper Industries Co., Ltd., 64 g/m²) was used as the sheets, and Test Chart No. 5-1 of the Imaging Society of Japan was used as a printing pattern. The printing operation was continuously performed to print the printing pattern on 250 sheets, and then the printed sheets were left on the stacking tray 9 for 5 minutes. It was then observed whether or not the above-described problems have occurred.

An image forming apparatus obtained by reconstructing a color multifunction machine (KM-C3232) manufactured by KYOCERA MITA Corporation was used in the experiment. The sheet conveying speed in the image forming section was set to 250 mm/s. The surface temperature of the fixing roller 7 a was set to 170 degrees C. The conditions under which the image forming process was performed were set such that the amount of toner applied to each sheet was 1.0 mg/cm² at a maximum.

An embodiment of the image forming operation will now be described. When print data (image information) is transmitted to the color printer 1 from a computer, the print data is divided into image data elements corresponding to the respective colors by a control unit of the color printer 1. The image data elements are subjected to suitable processes and are then transmitted to the respective exposure units 12. The surfaces of the photosensitive drums 10 are charged by respective charging units 11, and are subjected to the exposure processes performed by respective exposure units 12 in accordance with the image data elements. Thus, electrostatic latent images are formed on the surfaces of the photosensitive drums 10. The electrostatic latent images formed on the surfaces of the photosensitive drums 10 are developed by the respective developing units 13. Thus, toner images of the respective colors are formed. The toner images of the respective colors are successively transferred onto the sheet on the conveying belt 4 when the sheet passes through the sections between the photosensitive drums 10 for the respective colors and the first to fourth transfer rollers 5M, 5Y, 5C, and 5B. As a result, a color toner image is formed on the sheet. Then, heat and pressure are applied to the sheet when the sheet passes through the fixing unit 7, so that the color toner image is fixed on the sheet. The sheet on which the color image is formed is then fed by the plurality of pairs of conveying rollers 82 and the guide member 83. The sheet then passes through the discharge roller units 81 and is placed on the stacking tray 9.

The operation of the discharge section 8 will now be described. In the discharge section 8, when the first motor M1 is driven, the conveying rollers 82 start to rotate. Accordingly, the sheet on which the toner is fixed is fed by the plurality of pairs of conveying rollers 82 while being guided by the guide member 83. When the sheet approaches the pair of discharge roller units 81, the sheet is detected by a sensor (not shown) located near the discharge roller units 81. In response to the detection of the sheet, the second motor M2 is driven and the discharge roller units 81 start to rotate. When the second motor M2 is driven, the first discharge roller 181 starts to reciprocate in an axial direction. When the sheet is fed between the pair of discharge roller units 81, portions of the sheet are then deformed by the discharge roller units 81 into a recessed shape (when viewed from above). The sheet is then discharged.

More specifically, when the sheet on which the toner is fixed approaches the pair of discharge roller units 81 (the first discharge roller 181 and the second discharge roller 281), the driving operation of the second motor M2 is started. Accordingly, the second discharge roller 281 is rotated and the first discharge roller 181 is rotated due to the rotation of the second discharge roller 281. While the first discharge roller 181 and the second discharge roller 281 rotate, an end of the sheet is inserted into sections between the roller members 181 b of the first discharge roller 181 and the roller members 281 b of the second discharge roller 281. Accordingly, the end of the sheet is nipped between the roller members 181 b of the first discharge roller 181 and the roller members 281 b of the second discharge roller 281.

When the roller members 181 b and 281 b are rotated in this state, the sheet is deformed in a recessed shape (when viewed from above) due to the difference in hardness between the roller members 181 b and 281 b and the pressure of the contact force applied therebetween. The roller members 181 b and 281 b receive a predetermined pressure contact force and there is a predetermined difference in the hardness of the roller members 181 b and 281 b. Therefore, when the roller members 181 b and 281 b rotate, tracks H (passage marks) of the roller members 181 b in the first discharge roller 181 are formed on the sheet. Passage marks H are formed in a recessed shape when the sheet is viewed from above. In other words, passage marks H are formed as protrusions when the sheet is viewed from below. In addition, when the second motor M2 is driven, the first discharge roller 181 reciprocates in the axial direction. Therefore, passage marks H of the roller members 181 b are formed on the sheet such that passage marks H are curved (see FIG. 3).

Thus, the first discharge roller 181 is reciprocated in the axial direction while the first discharge roller 181 and the second discharge roller 281 are rotated around the axes thereof, so that passage marks H of the roller members 181 b are formed on the sheet in a curved shape, as shown in FIG. 3. When the entire area of the sheet leaves the pair of discharge roller units 81, the sheet on which the curved recesses (or projections) H formed thereon is discharged to outside the apparatus and is placed on the stacking tray 9.

In the color printer 1, when the sheet on which the toner is fixed is fed by the first discharge roller 181 and the second discharge roller 281, the sheet is deformed in a recessed shape (when viewed from above). More specifically, when the roller members 181 b of the first discharge roller 181 and the roller members 281 b of the second discharge roller 281 are rotated, the passage marks H of the roller members 181 b are formed on the sheet in the recessed shaped due to the difference in hardness between the roller members 181 b and 281 b, which face each other. At the same time, the first discharge roller 181 is reciprocated in the axial direction, so that passage marks H of the roller members 181 b are formed on the sheet in a curved shape. Thus, passage marks H of the roller members can be formed on the sheet in a curved and recessed shape when the sheet is viewed from above.

Therefore, even if the heated sheets are discharged to the stacking tray 9 and stacked on top of each other, since the sheets are partially deformed due to the recessed shape, as shown in FIG. 4, spaces are provided between the sheets due to the deformation of the sheets. Therefore, heat from the sheets can be dissipated into the surrounding space through the spaces provided between the sheets. FIG. 4 is a schematic diagram illustrating the state in which the sheets having portions deformed in a recessed shape are stacked on the stacking tray 9.

Passage marks H of the roller members are formed on the sheets in a recessed shape at different positions for each sheet when the sheets are fed between the roller members 181 b and 281 b. Therefore, spaces can be formed between the sheets due to the deformation of the sheets. As a result, the heat of the sheets can be dissipated through the spaces provided between the sheets on top of each other. Thus, in the color printer 1, the temperature of the sheets can be reduced without using a special component or device.

In the above-described embodiment, the roller members 181 b and 281 b are attached to the shaft members 181 a and 281 a, respectively, such that the shaft members 181 a and 281 a extend through the centers of the roller members 181 b and 281 b (centers of the cylinders). However, the manner in which the roller members 181 b and 281 b are attached to the shaft members 181 a and 281 a, respectively, is not limited to that in the above-described embodiment.

For example, the roller members 181 b and 281 b may also be attached to the shaft members 181 a and 281 a, respectively, in a non-rotatable manner such that the shaft members 181 a and 281 a extend through positions other than the centers of the roller members 181 b and 281 b. In other words, as shown in FIG. 5, the roller members 181 b and 281 b may be eccentrically attached to the shaft members 181 a and 281 a, respectively, in a non-rotatable manner.

In this situation, effects similar to those of the above-described embodiment can be obtained. In addition, in this situation, the depth of the recessed portions formed in the sheet can be varied when the sheet is fed between the roller members 181 b of the first discharge roller 181 and the roller members 281 b of the second discharge roller 281. Accordingly, when the fed sheets are stacked on top of each other, the spaces can be formed between the sheets. Even when the heated sheets are placed on top of each other, the heat of the sheets can be dissipated through the spaces provided due to the deformation of the sheets.

Table 3 shows the result of an experiment in which the amount of eccentricity of the roller members 181 b, the maximum depth (μm) of the recessed portions formed in the sheets, and stacking performance were observed. In the experiment, the distance between the axial centers of the shaft member 181 a and the shaft member 281 a was set such that when the surfaces of the portions of the roller members 181 b which are closest to the axial center of the shaft member 181 a come into contact with the surfaces of the roller members 281 b, a force of 0.2 N is applied to the pressure contact surfaces of each pair of roller members 181 b and 281 b which are in contact with each other. The depths of the recessed portions in each sheet were measured by a method similar to that used in the previous embodiment.

FIGS. 7A and 7B are schematic diagrams illustrating the recessed portions formed in the sheets in the experiment. FIG. 7A is a schematic diagram illustrating a cross section of one of the recessed portions in the sheet taken along the thickness direction of the sheet. FIG. 7B is a perspective view of the recessed portions formed in the sheet viewed obliquely from above.

As shown in Table 3, the maximum depth of the recessed portions which can be formed in the sheet without causing an issue is about 40 μm. Since the recessed portions have a gently inclined surface as shown in FIG. 7A, the recessed portions are visually indiscernible. Therefore, the print quality is not affected by the recessed portions formed in the sheet.

As a result of the above-described experiment, it was determined that sufficient stacking performance can be obtained when the amount of eccentricity is set in the range of 0.2 mm to 0.8 mm. In this experiment, settings were made such that the depth of the recessed portions is zero when the amount of eccentricity is zero. In this experiment, VM (Nippon Paper Industries Co., Ltd., 64 g/m²), XeroxC2 (Oji Paper Co., Ltd., 70 g/m²), and CC90 (MONDI, 90 g/m²) were used as the sheets. In addition, Test Chart No. 5-1 of the Imaging Society of Japan was used as an image to be formed on the sheets. Other conditions of this experiment were similar to those in the experiments described in the first embodiment.

TABLE 3 MAXIMUM PROBLEM IN SHEET TYPE ECCENTRICITY DEPTH STACKING VM (NIPPON PAPER 0.2 mm 15 NONE INDUSTRIES) 0.5 mm 28 NONE 64 g/m2 0.8 mm 40 FEW 1.1 mm 55 MANY XeroC2 (OJI PAPER) 0.2 mm 12 NONE 70 g/m2 0.5 mm 23 NONE 0.8 mm 38 NONE 1.1 mm 50 MANY CC90 (MONDI) 0.2 mm 7 NONE 90 g/m2 0.5 mm 15 NONE 0.8 mm 29 NONE 1.1 mm 37 NONE

Table 4 shows the result of an experiment in which the occurrence of sticking of the sheets and offsets were described in relation to the amount of eccentricity of the roller members 181 b. In this experiment, the amount of eccentricity of the roller members 181 b was set in the range of 0.2 mm to 0.8 mm on the basis of the experimental results shown in Table 3. More specifically, in the experiment, the amount of eccentricity of the roller members 181 b was set to 0.2 mm, 0.5 mm, and 0.8 mm.

From this experiment, it was determined that sticking of the sheets and the offset can be prevented without reducing the stacking performance of the sheets when the amount of eccentricity is in the range of 0.5 mm to 0.8 mm. Thus, in the situation where the roller members 181 b are eccentrically attached, the amount of eccentricity is preferably in the range of 0.5 mm to 0.8 mm.

TABLE 4 SHEET TYPE ECCENTRICITY STICKING OFFSET VM (NIPPON PAPER 0.2 mm FEW MANY INDUSTRIES) 0.5 mm NONE NONE 64 g/m2 0.8 mm NONE NONE XeroC2 (OJI PAPER) 0.2 mm FEW MANY 70 g/m2 0.5 mm NONE FEW 0.8 mm NONE NONE CC90 (MONDI) 0.2 mm MANY MANY 90 g/m2 0.5 mm NONE FEW 0.8 mm NONE NONE

In Table 3 and 4, “NONE” means that no problems occurred in any of the sheets, “FEW” means that no problems occurred in 98% or more of the sheets, and “MANY” means that problems occurred in more than 2% of the sheets.

According to the previous embodiments, the distance between the two shaft members 181 a and 281 a is constant. However, the distance between the two shaft members 181 a and 281 a may be varied so that the depth of the recessed portions (or the height of the projections) varies when the sheet is fed between the roller members 181 b of the first discharge roller 181 and the roller members 281 b of the second roller 281. Accordingly, when the fed sheets are stacked on top of each other, the spaces can be formed between the sheets due to the deformation of the sheets. When the heated sheets are stacked on top of each other, the heat of the sheets can be dissipated through the spaces provided when the sheets are partially deformed so as to have a recessed shape.

A mechanism for varying the distance between the two shaft members 181 a and 281 a will be described with reference to FIG. 8. The shaft member 181 a of the first discharge roller 181 is inserted through long holes 101 a formed in the side plates 101 with bearings 181 c provided therebetween. The shaft member 181 a is rotatably retained by the side plates 101 while the shaft member 181 a is urged toward the axial center of the second discharge roller 281 by urging springs 300′. The second discharge roller 281 is rotatably supported by the side plates 101 so that the position of the second discharge roller 281 is fixed with respect to the side plates 101. A drive gear 311′ is fixed to an end portion of the shaft member 281 a of the second discharge roller 281. The drive gear 311′ meshes with an output gear 310′ of the motor M2, so that the rotation of the motor M2 is transmitted to the drive gear 311′. Eccentric cams 312′, which serve as eccentric rotating members, are attached to the shaft member 281 a at the ends thereof in a non-rotatable manner such that the eccentric cams 312′ are in the same phase. The eccentric cams 312′ are in contact with the bearings 181 c on the shaft member 181 a of the first discharge roller 181 at positions corresponding to the positions at which the shaft member 181 a is urged by the urging springs 300′.

When the eccentric cams 312′ are rotated in synchronization with the rotation of the second discharge roller 281, the distance between the axial centers of the first discharge roller 181 and the second discharge roller 281 periodically changes due to the rotation of the eccentric cams 312′. Accordingly, the nip depth between the roller members 181 b and 281 b varies, and the depth of the recessed portions formed in the sheet which passes through the roller members 181 b and 281 b varies accordingly. As a result, the above-described effects can be obtained.

In the above-described embodiments, it is also possible that the sheet is recessed toward the roller members 181 b of the first discharge roller 181 when the sheet is fed between the first discharge roller 181 and the second discharge roller 281 by changing of the arrangement of the thrust mechanism or the eccentric cams 312′, of the hardness or the width of the roller members 181 b, 281 b.

In the above-described embodiments, a color printer is used as an example of an image forming apparatus. However, the present invention may also be applied to other types of image forming apparatuses, such as a color copy machine. In addition, the present invention can also be applied to an inkjet recording apparatus to prevent undried ink on a sheet discharged to a sheet discharge section from adhering to the back side of a sheet stacked thereon.

It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims. 

1. A discharge device comprising: a discharge roller unit including a first discharge roller and a second discharge roller, the discharge roller being so constructed and arranged that a sheet on which an image is formed is deformed such that the sheet is recessed toward one of the rollers when the sheet is fed between the first discharge roller and the second discharge roller.
 2. The discharge device according to claim 1, wherein the first discharge roller reciprocates in an axial direction.
 3. The discharge device according to claim 2, wherein the first discharge roller is urged in a direction from a first end to a second end of the first discharge roller and an end portion of the first discharge roller, located at the second end, is slid in the axial direction.
 4. The discharge device according to claim 3, wherein the end portion of the first discharge roller is in contact with a surface having a height in the axial direction that continuously varies.
 5. The discharge device according to claim 2, wherein the first discharge roller reciprocates in the axial direction at a period that is different from a period of a rotation at which the discharge roller unit feeds the sheet.
 6. The discharge device according to claim 1, wherein the first discharge roller includes a shaft and a plurality of roller members attached to the shaft in a non-rotatable manner, the roller members being capable of nipping the sheet.
 7. The discharge device according to claim 6, wherein the second discharge roller includes a shaft and a plurality of roller members attached to the shaft in a non-rotatable manner, the roller members being capable of nipping the sheet, a hardness of the roller members in the first discharge roller differs from the hardness of the roller members in the second discharge roller.
 8. The discharge device according to claim 7, wherein the hardness of the roller members in the first discharge roller is greater than the hardness of the roller members in the second discharge roller.
 9. The discharge device according to claim 6, wherein the roller members are eccentrically attached to the shaft.
 10. The discharge device according to claim 1, wherein the first discharge roller is so constructed and arranged that a distance between axes of the first discharge roller and axes of the second discharge roller is varied.
 11. The discharge device according to claim 10, wherein the first discharge roller is supported by bearings at the ends of the first discharge roller, the bearings being in contact with eccentric rotating members.
 12. The discharge device according to claim 1, wherein the discharge roller being so constructed and arranged that a sheet on which an image is formed is deformed such that the sheet is recessed toward the second discharge roller when the sheet is fed between the first discharge roller and the second discharge roller.
 13. An image forming apparatus comprising: a sheet feeding section which stores and feeds a sheet; a conveying section which feeds the sheet; an image forming section which forms an image on the sheet based on the image information; and a discharge device for discharging the sheet on which the image is formed comprising a discharge roller unit including a first discharge roller and a second discharge roller being so constructed and arranged that the sheet on which the image is formed is deformed such that the sheet is recessed toward the second discharge roller when the sheet is fed between the first discharge roller and the second discharge roller.
 14. An image forming apparatus comprising: a conveying section which feeds a sheet of paper; an image forming section which forms an image on the sheet of paper; and a discharge device for discharging the sheet of paper comprising a discharge roller unit including a first discharge roller and a second discharge roller, the discharge roller being so constructed and arranged that the sheet of paper is deformed such that a portion of the sheet is recessed toward one of the rollers when the sheet is fed between the first discharge roller and the second discharge roller. 